Battery packs can be used with a variety of devices. These devices include power tools. Power tools are of many types. Examples of power tools include drills, drill/drivers, hammer drill/drivers, rotary hammers, screwdrivers, impact drivers, circular saws, jig saws, reciprocating saws, band saws, cut-off tools, cut-out tools, shears, sanders, vacuums, adhesive dispensers, concrete vibrators, staplers, nailers, flashlights, radios, and lasers.
Power tools typically receive electrical power from a wall outlet or from the battery pack that is removably coupled to the power tools. The battery pack may include one or more electrochemical cells (hereinafter “cells”). The cells can be of different types. For example, the battery pack may include nickel-cadmium (NiCd) cells or lithium-ion (Li-ion) cells.
The power tools typically include a motor and a switch that is used to vary the speed of the motor. During variable speed mode, the switch is typically turned on and off (closed and opened) using pulse width modulated signals. The speed of the motor is varied by varying a duty cycle of the pulse width modulated signals. The circuitry that generates the pulse width modulated signals may be included in the tool portion or the battery back portion of the power tool. Some power tools may not have the variable speed feature and may simply have on/off mode.
One of the electrical characteristics of the cells is that each cell has an inductance. The value of the inductance depends on the chemical composition and mechanical construction of the cell. Due to the inductance of the cell, an inductive energy builds up in the cell while the switch is closed. When the battery or tool switch is turned off, the inductive energy typically dissipates across the tool or battery switch and heats the switch. Depending on the value of the inductance, the frequency and the duty cycle at which the switch is turned on and off by the pulse width modulated signal, the heat generated by the inductive energy can cause the switch to malfunction.
Therefore, in order to prevent the switch from heating and malfunctioning, it is desirable to provide a protective device that will divert the inductive energy from the cells of the battery pack. The protective device can prevent the inductive energy from dissipating across the switch, thereby preventing the switch from malfunctioning.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.